Objectives To determine the metabolites of testosterone in the urine and feces of male cotton-top tamarins in order to accurately assess sexual maturation and social effects on male fertility. ABSTRACT:Before we can understand the social effects on fertility in male cotton-top tamarins, we need to know which steroid metabolites are reflecting testosterone excretion into the urine. In several New World species estrogens appear to be produced in very high levels in males and may be metabolites of testosterone. I.M. injections of 500ng or 1 g of testosterone into 6 males versus 2 controls indicated that testosterone, dihydrotestosterone and androstenedione are all metabolites of testosterone in male cotton-top tamarin urine with testosterone being the predominate of these steroids. Testosterone is found in the conjugated form in urine with over 90% found as complex conjugates only released after solvolyses. HPLC separation of male tamarin urine and identification of steroids by UV detection indicated that two additional steroids increased post injection that have yet to be identified. Both estrone and estradiol are found in large levels in the urine of male tamarins but do not appear to be a metabolite of testosterone. Current experiments on shutting down the testes by the use of Antide, a long-acting GnRH antagonist, are underway and will provide useful information on determining the source of the estrogens in the urine. Keywords testosterone metabolites, urine, feces